This part will get you through the start of your quest.
The story starts with Niko the pirate giving us (and Link) a brief slideshow recap of the events that took place in The Wind Waker, to which Link sleeps through. Tetra calls Link to attention to be on the lookout for the mysterious Ghost Ship, which she believes is actually a bunch of pirates terrorizing the seas and that may be loaded with treasure. Soon the Ghost Ship does appear, and Tetra leaps aboard, only to scream as she is taken captive. Link leaps toward the Ghost Ship and tries to pull himself aboard to rescue Tetra, but ends up falling into the water as it disappears into the mist. In his dream he sees Tetra calling out to him as she slips into a light swallowed up by a purple mist.
Link wakes up to a voice calling him. It sounds like Navi from Ocarina of Time, but is in fact Ciela, a fairy who found the young boy washed ashore. She finds out from Link what had happened and tells him what she knows about the Ghost Ship -- only that those who are captured by the ship are never found again. She leads Link to follow her to her "grandpa's" house, where he might know more about the Ghost Ship that took Tetra.
Mercay Island Village
Mercay Island Village is the starting point of our adventure. We're going to go up and to the northeast to Grandpa's house. Here an old man with a staff named Oshus greets Link, who warns the boy about not going after the Ghost Ship. However, Ciela is adamant that they must do something to help Link rescue his friend. Oshus realizes that Link will stop at nothing to rescue her and tells him there's a port on the east end of the island where a captain named Linebeck is located, who could probably know more about the Ghost Ship. Ciela knows who Linebeck is and offers to go with Link to the port to find him. Oshus relents and allows her to go, but then warns Link that there are monsters moving about.
Leave Oshus' house and head toward the right. As we do, an earthquake takes place, and as we head toward the right where the river is and talk to the lady, we see that the earthquake has broken the bridge, so we can't get to the port going that way. Head back to the left and go up north on the path where the sign is. We'll see Red Chuchus roaming about, and Ciela will tell us that we need something to defend ourselves with. Head back to Oshus' house, and he will tell us that the path to the cave up north will take us to the port. He will warn us not to go that way and to wait for someone to repair the bridge.
Unfortunately, we can't wait, so as soon as leave Oshus' house, head for the cave to the immediate right, tossing aside the barrels that are blocking it, and head inside. Once inside the cave, Ciela says that Grandpa uses it as a storehouse and that he might have a sword stashed away. There's a door in the cave that we need to open. Ciela says Grandpa writes a number on the sign next to the door to open it, saying that it's the number of palm trees on the beach. You can go outside and check it for yourself, then write the number on the sign when you come back. With the correct number, the door will open, and we can go across the bridge toward the chest which contain's Oshus' Sword.
Go back outside, and Oshus will meet us outside, wondering what we intend to do with that sword. However, Oshus won't let us go until he puts us through a tutorial on how to use the sword. Go through the tutorial, and we can finally venture through the path that will take us to the port on the other side of the island.
Before you take the path, you could go over to the man's house all the way to the left and clear away the rocks in front of his house. All you'll get for it is a rupee, but he will tell you the location of a tree on the island that you can bash really hard, which you can mark on your map.